Datura family: the 2009 update

Context. Research of asteroid families has been recently refreshed by the discovery of very young ones. These families are of great interest because they represent the product of their parent body fragmentation before orbital and physical evolutionary processes can change them. A cluster of seven objects around the largest body (1270) Datura is of particular interest because it has enough known members and resides in the inner part of the main asteroid belt, facilitating observations. Aims. We carried out photometric observations of the two largest members of the Datura family - asteroids (1270) Datura and (90265) 2003 CL5 ― with the goal of inferring their physical parameters. We also used numerous astrometric observations of Datura-family members in the past few years to revisit the age of this cluster. Methods. We performed numerous photometric observations of (1270) Datura over several oppositions. We then used the lightcurve inversion method to determine the spin state and shape of this asteroid. In the case of (90265) 2003 CL5, for which only limited lightcurve data have been acquired so far, we used Fourier analysis to determine the synodic rotation period during the 2008 apparition. We also used backward numerical integration of the improved orbits of Datura family members to reduce uncertainty in its age. Results. We determined the rotation state of (1270) Datura, the largest member of its own family. Its major properties are a short rotation period of ∼3.36 h and small obliquity, which, however, exhibits ∼±15° excursions because of a forced Cassini state of the proper nodal frequency. Any possible initial non-principal rotation state has probably been damped and the asteroid rotates about the shortest axis of the inertia tensor. Its global shape, although convex in our representation, may reflect regions related to the excavation of the family members from the parent body surface. Interestingly, the second largest member of the Datura family -(90265) 2003 CL5 ― appears to be very slow rotator with the rotation period ∼24 h. The large amplitude of its rotation curve suggests that its shape is extremely elongated, possibly bi-lobed. Improved orbits of the family members allow us to re-determine the possible age of this family. We find an age that is slightly older than previously reported. Using a conservative approach, we obtain an age in the 450 to 600 kyr range. With strengthened, but plausible, conditions, we find that the current data may support an age of 530±20 kyr. Further astrometric and photometric observations of the Datura cluster members are needed to determine its age more accurately.